AVR Hall Sensor

#include <avr/io.h>
#include <avr/interrupt.h>
#include <avr/pgmspace.h>
// #include <avr/eeprom.h>

#include <util/delay.h>
#include <stdlib.h>
#include <string.h>
#include <math.h>

#include "lcd.h"
// #include "spi.h"


#define clockCyclesToMicroseconds(a) (((a) * 1000L)/(F_CPU / 1000L))
#define MICROSECONDS_PER_TIMER0_OVERFLOW (clockCyclesToMicroseconds(64 * 256))
#define MILLIS_INC (MICROSECONDS_PER_TIMER0_OVERFLOW / 1000)
#define FRACT_INC ((MICROSECONDS_PER_TIMER0_OVERFLOW % 1000)>>3)
#define FRACT_MAX (1000>>3)


volatile float value = .0, rev = .0, last = .0;
volatile uint16_t rpm = 0, count = 0;
volatile uint16_t oldTime = 0, time = 0;
volatile uint8_t counter = 0;


volatile unsigned long int timer0_overflowCount = 0;
volatile unsigned long int timer0_millis = 0;
static unsigned char timer0_fract = 0;
unsigned long int starttime = 0, endtime = 0;

unsigned long int millis() {
    unsigned long int m = 0;
    uint8_t oldSREG = SREG;

    // Disable interrupts while we read timer0_millis or we might get an
    // Inconsistent value (e.g. in the middle of a write to timer0_millis)
    cli();
    m = timer0_millis;
    SREG = oldSREG;
    return m;
}


int main() {
    // VSS signal, injector signal and navigation buttons - Atmega 8
    DDRD &= ~((1<<PD3) | (1<<PD2));  // PD2/INT0 and PD3/INT1 input
    PORTD |= ((1<<PD3) | (1<<PD2));  // PD2/INT0 and PD3/INT1 internal pull-up resistor

    MCUCR |= ((1<<ISC01) | (1<<ISC00));  // RISING edge of INT0 generates interrupt
    GICR |= (1<<INT0);


    // 16 bit timer for VSS
    TCCR1A = 0;  // Timer1 normal mode
    TCCR1B |= ((1<<CS10) | (1<<CS11));  // Prescaler 64, causes overflow every 0.524288 (8 MHz crystal, 0.262144s for 16 MHz)
    TCNT1 = 34286;  // Counts from 34286 to 65535 - causes overflow every 0.25s (8 MHz crystal, 3036 for 16 MHz)

    // Counter for millis() function
    TCCR0 |= ((1<<CS01) | (1<<CS00));  // Prescaler 64
    TCNT0 = 0;  // Counts from 0 to 255;

    // Enable timer overflow interrupt
    TIMSK = ((1<<TOIE1) | (1<<TOIE0));


    char buffer[16];
    LCD.init();

    sei();  // Enable global interrupts
    while(1) {
        LCD.cursor(1, 1);
        LCD.sends("RPM: ", 1); LCD.sends(itoa(rpm, buffer, 10), 1);

        LCD.render();
        LCD.clear();
    } return 0;
}


ISR(TIMER0_OVF_vect) {
    unsigned long int m = timer0_millis;
    unsigned char f = timer0_fract;

    m += MILLIS_INC;
    f += FRACT_INC;
    if(f >= FRACT_MAX) {
        f -= FRACT_MAX;
        m += 1;
    }

    timer0_fract = f;
    timer0_millis = m;
    timer0_overflowCount++;
}

ISR(TIMER1_OVF_vect) {
    counter++;

    if(counter > 3) {
        cli();
        time = millis()-oldTime;
        rpm = (rev/time)*60000;
        oldTime = millis();
        rev = 0;
        sei();
    } TCNT1 = 34286;
    last = rev;
}


// Hall signal interrupt
ISR(INT0_vect) {
    rev++;
    count++;
}